Riyad as-Salihin 17:187

Abu Hurairah (May Allah be pleased with him) said:

The Messenger of Allah said, "When you see someone buying or selling in the mosque, say to him: 'La arbaha-Allahu tijarataka (May Allah not make your bargain profitable)!' When you see someone announcing something lost in it, say: 'May Allah not restore it to you!"' [At-Tirmidhi] .